Contrary to popular belief, the spelling of your as yr did NOT start with Sonic Youth. It actually started with the Beat Generation and the likes of Jack Kerouac and Allen Ginsberg. Kerouac spelled your as yr in his 30 essentials for spontaneous prose.

Contrary to popular belief, not coined or made popular by indie favorite Sonic Youth. Also, NOT coined by the Beat Generation poets such as Kerouac or Ginsberg. Shorthand for "your" (or "you're), my first encounter with this usage dates back to 1818 in a John Keats letter addressed to John Hamilton Reynolds.

"Yr* sincere friend and Coscribbler

John Keats."

*"r" is actually elevated and smaller than the rest of the text. Example: The 30(^)th.